NASA has been researching new technology and system concepts to meet the requirements of aeropropulsion for 21st Century aircraft. The air transportation for the new millennium will require revolutionary solutions to meet public demand for improving safety, reliability, environmental compatibility, and affordability. Whereas the turbine engine revolution will continue during the next two decades, several new revolutions are required to achieve the dream of an affordable, emissionless, and silent aircraft. This paper reviews the continuing turbine engine revolution and explores the propulsion system impact of future revolutions in propulsion configuration, fuel infrastructure, and alternate energy systems. A number of promising concepts, ranging from the ultrahigh to fuel cell-powered distributed propulsion are also reviewed.